1. Field of the Invention
The present invention concerns a method for monitoring a radio frequency power amplifier, as well as a corresponding radio-frequency device, a corresponding radio-frequency monitoring device and a corresponding magnetic resonance tomography system.
2. Description of the Prior Art
Magnetic resonance tomography is an imaging modality now in widespread use, which is based on the detection of signals arising from precessing nuclear spins of protons in a body region of an examination subject. First, a strong, stable homogenous magnetic field is generated in which the body region is disposed, which causes a stable alignment of the protons in the body region. This stable alignment is altered by radiating electromagnetic radio frequency energy into the region. After this excitation the magnetic resonance signals created in the body are detected with suitable receiver coils. The signals are processed are processed and an image of the tissue in this body region is reconstructed therefrom, with which a medical diagnosis can be made, or a surgical procedure can be planned or guided.
A magnetic resonance tomography system has a number of interacting components, each of which requires the use of modern and elaborate technologies. A central component of a magnetic resonance tomography system is the radio frequency device. This is responsible for the generation of the radio frequency pulses that are radiated into the body region to be imaged. The radio frequency pulses at the output of a radio frequency power amplifier of a magnetic resonance tomography system are conducted via a measurement device to a transmission coil that radiates the radio frequency pulses into the body region. As used herein “transmission coil”, means any antenna device with which the radio frequency pulses can be radiated.
With the development and establishment of magnetic resonance tomography systems, limit values to ensure patient safety have been standardized that regulate the maximum permissible radio frequency irradiation into a human body. A typical limit value for this is the maximum allowable SAR value (SAR=specific absorption rate).
To abide by these limit values, in the measurement device cited above, measurement values are recorded that represent the power of the radio frequency pulses radiated by the transmission coil. Control values are formed on the basis of a number of such measurement values. These control values are then compared with a fixed threshold (limit control value) that is predetermined by a standard, and the radio frequency power amplifier is automatically limited in operation (usually deactivated) if and when a control value exceeds the predetermined threshold.
The maximum radio frequency power that the patient safely tolerates—without health impairment—is dependent on, among other things, different environmental parameters. In particular, a direct relation exists between the maximum radio frequency power and climate values, for example the temperature and/or the humidity of the immediate environment of the patient. For this reason, new guidelines for the SAR monitoring require adaptation of the SAR limit values to the climate values of the environment of the patient. However, since the temperatures in the examination region change continuously, the limit values would then have to be correspondingly continuously adapted during a running data acquisition.
In conventional monitoring, an average value of the radio frequency power radiated in the appertaining time window is determined based on the sum of a number of values measured in a preceding time window. In particular in connection with magnetic resonance tomography systems, such a value is a good measure for the total radiation exposure to which an irradiated person has been exposed in the previous time window. This power average is then compared with a fixed limit value for the appertaining, retroactive limit value. A simple monitoring of the adherence to the limit values is possible in this manner.
For limit values that continuously change, however, such an inflexible monitoring concept is no longer suitable. The problem is that, as soon as a new limit value is determined in the examination space (for example based on a temperature and/or humidity measurement), this limit value must assumed to have been effective for a past time window in order to be able to ensure complete safety. In practice, in conventional methods this would necessitate the radio frequency power amplifier always having to be deactivated when, for example, the limit value is decreased due to a temperature increase. This can be explained in a simple example: Assume that, given a temperature of 25° C. and 60% humidity, the maximum limit value for the radio frequency power is at 4 W/kg of body weight of the patient at a first point in time t1. Furthermore, assume that for the current time period this power limit value is used and the power is radiated in the extreme case up to 4 W/kg. It should be noted that generally it is reasonable to operate the system with an optimally high radio frequency power within the allowable safety limit values, since this leads to a higher dynamic range and consequently to an improvement in the quality of the generated exposures. At a subsequent measurement point in time t2, it is then established that a temperature of 33° C. exists with 60% humidity. The maximum allowable power value is then only 2 W/kg. However, since a limit value of 4 W/kg has been assumed in the time span between the first measurement at point in time t1 and the second measurement point in time t2, the probability is very high that this limit value has already been exceeded at the point in time t2 of the determination of the new limit value. Therefore, the apparatus would have to be deactivated immediately due to this retroactive consideration. Thus with conventional methods, it is usually not possible to monitor a number of measurements in succession with constantly changing (for example, due to changing environment parameters) limit values.